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You head to a bar one night in an area that’s completely foreign to you, in a state you’ve never visited. As you walk in, you decide to get a martini and order from the barkeep. Then you turn around and notice, standing in the corner, a drag queen in a pink leotard holding a fiddle. Suddenly, you get a shiver done your spine, as this all seems overwhelmingly familiar. Too familiar. Unless this is one of those rare jet-setting drag queen troubadours, there’s no way this could have happened before. But you swear it has. You, my friend, have a major case of déjà vu.

What is déjà vu? Hint: It’s not just your favorite Crosby, Stills, Nash, & Young album. Déjà vu is essentially an experience where there is an unexplainable sense of recognition, but with no real pretext or awareness for this genuine yet indescribable feeling. The term was first coined by French psychic Émile Boirac and literally means “already seen.” Since it was first talked about in the 1800s, neurobiologists have worked hard to contend with this ephemeral topic; there seems to be over fifty theories floating around about the cause of déjà vu. It’s not just a phenomenon for Frenchies. What makes déjà vu such a spooky, unsettling occurrence is that it can’t be verified or observed objectively, but it still remains a common and almost universally reported experience.

Don’t fret. There are some things about déjà vu that we do know for certain. About 70% of the population has reported experiencing it at least once, but in many cases multiple times in their life.

Who experiences it the most? An ideal candidate would be a young Ralph Nader supporter sipping a beer after work with pals. This is because it most commonly occurs with people ages 15 to 25, with those that remember their dreams, and randomly enough, with those of liberal values. We get it often during mundane activities, when we’re with friends, when we’re stressed out or tired, and at night. Often it is triggered through physical perceptions, rather than words, and lasts from about 10 to 30 seconds. It’s not just visual, either; there are plenty of reports that blind people experience déjà vu.

There are three types of recognized déjà vu experiences, categorized by psychologist Arthur Funkhouser. These three can take place in isolated experiences or altogether, like a déjà vu combo pack. Déjà vecu — meaning “already experienced” — is the one you’ve probably had the most. It usually consists of the feeling of having already been in a situation. The feeling is derived through the senses and our perception of our body’s positioning and often involves normal day activities.

Déjà visité, though more rare, is the feeling of already having visited a new place. Spatial associations rather than situational are key triggers here.

The last type, déjà senti, or already felt, is trickier to explain. Often the experience feels like we are discovering missing information and involves a fulfillment of having recovered a memory, but without the aid of a creepy hypnotist. There is no premonition quality to this déjà vu, and though these episodes are longer than a few seconds, the recovered feelings are fleeting. Déjà senti is associated with seizures experienced by patients with temporal lobe epilepsy. Some epileptics experienced déjà vu auras right before temporal lobe seizures, so déjà vu is now linked with that part of the brain, the area that deals with familiarity. Does that mean every time we get an eerily familiar feeling we are all having mini seizures? Maybe. Maybe not.

Science has been trying to make sense of this amorphous tangle for years. One neurobiological explanation for déjà vu is that we are constantly taking in different tiny bits of sensory data through different neuronal pathways in order to create one whole experience or one whole picture. If by some malfunction, the speed of one of these data cars is altered on their highways, they don’t reach the same processing center at the same time. Thus, the effect, even if they are meeting mere milliseconds apart from one another, can be that the brain interprets the data as two identical and yet separate experiences. We think of memories and perceptual experiences like one whole candy bar, but we should be thinking of them like M&Ms. Memory is actually fractured, and tiny morsels of sensory input complete our delicious understanding of things.

Because perception works like a puzzle, seeing things out of the corner of our eye, in the periphery, can lead to déjà vu. Think back to the drag-queen-in-bar scenario. You enter the bar talking to your friend, immediately noticing the bartender at his post. While making a beeline for the bar, you pass by the fiddling drag queen, but you don’t process seeing her. You know you saw the bartender and stored him immediately in your memory. However, though you don’t know that you saw the drag queen, she was stored in your memory just the same. So, when you finally come back around and consciously notice her — aha — you might feel like you have déjà vu, like you have seen it somewhere long ago. While you have just subconsciously ignored sensory data, the memory seems to come from long term storage.

Déjà vu could also deal with our recognition memory, which can be split into two types: recollection and familiarity. Familiarity memory, a kind of knowing for which we cannot find the explicit source, is thought to be connected to déjà vu. You know when you come to a place you’ve never been and you feel like you have been there before, but you know you actually haven’t? That could be because one element of the current situation is nearly identical to an element in a previous experienced situation that we’ve subconsciously sponged up. This small element could be from waking life, dreams, or from fictions like books or movies–literally almost anything. The tiniest fracture of a memory, whether a word, object, or spatial configuration, that we don’t have an overt source for, could then whitewash the entire new experience. We then get a sense that it had previously happened, as long as one of those memories is somehow perceived to be repeated. Let’s hope that the next sensory item triggering your déjà vu isn’t from “The Shining.”

There are a few other voices chiming in the déjà vu debacle. Some say déjà vu could be a side effect of excessive neurotransmitters, like dopamine, found in the brain. It can be promoted by drugs like clonazepam, alcohol, and amphetamines. But, don’t pop an Adderall just yet. Psychoanalysts say déjà vu is just a manifestation of our need for wish fulfillment and that we are constantly living through past experiences with a search for more positive outcomes. Well, that’s depressing. Though, it makes sense if déjà vu is most common for the hippie daydreamer, after all.

And on yet another side of the ring, parapsychology suggests that déjà vu is proof that reincarnation is real. A feeling of knowing what will happen next, sometimes experienced with déjà vu, is their main evidence, but this is a little dubious. We can’t forget that this experience, though a mystery, is a neurological malfunction at its heart.

Okay already, but will we ever know the cause? Epileptic patients could be a key to unlocking this cognitive illusion. Electrical recordings of déjà vu activity in the brain, only possible by studying preseizures, could tell us more about where the heck it comes from and why it happens. What’s stalling research is that we can’t really predict when it will happen, where it will happen, or who it will happen to.

It may be unnerving to researchers, but most people feel positively about their déjà vu. It’s sort of deviously gratifying to feel familiarity when we shouldn’t. Whether we are a reincarnated queen, and that’s why we remember the shape of Windsor castle, or our brains just freeze like our computers do, there’s still plenty to explore about this enigma. And unless we feel like we’re dating our ex over and over again, déjà vu isn’t doing us any harm. Even if it is just a temporal-sensory perception hiccup, it makes the human mind just that much more mystical.

And anyways, no matter what research says, we all know what déjà vu really means. Just like Keanu, we are only experiencing a glitch in the Matrix.

ExpliKate is a column running on Kate-book.com every other Monday at noon. It is written by the hyper-inquisitive Kate Hakala, who seeks to answer questions ranging from “What is kombucha?” to “Can guys have an orgasm without ejaculating?” Follow her on Twitter here. While this column is a repeat of a 2012 favorite, stay tuned in the new year for brand new pieces.

[...] There are so many fascinating Kates out there. Every week, Kate-book.com is resolving to introduce you to a new one. This week, I would like you to meet Kate Hakala, 24, of Brooklyn, NY—who you know a bit from her column ExpliKate, where she’s explained everything from uncontrollable tears to deja vu. [...]

I get déjà vu all the time! In fact, all three types. For me though, the scientific explanations don’t always make sense because sometimes I can pinpoint that I felt the sensation or the place in a specific period in the past. I’ll know that I dreamt it or said it several years ago. It’s so eerie. Can’t wait to read your next installment.